Neurotensin (NT) is a thirteen amino acid peptide, isolated from bovine hypothalamus and has the following structure: pGlu-Leu-Tyr-Glu-Asn-Lys-Pro-Arg-Arg-Pro-Tyr-Ile-Leu-OH (SEQ ID NO:1) wherein pGlu is pyroglutamate. High concentrations of neurotensin receptors are found in discrete regions of the mammalian central nervous system including the brain and in the gut. In addition, neurotensin receptors are found in several tumor cells, including small cell lung carcinoma, exocrine pancreatic cancer (Reubi et al., 1998), Ewing sarcoma, meningiomas, medulloblastomas and astrocytomas. Normal pancreatic tissue and tissue from patients with pancreatitis or endocrine pancreatic cancer do not express neurotensin receptors (Reubi et al., 1998). It is estimated that there are 58,000 cases of exocrine pancreatic cancer per year in the United States and Europe. The five year survival rate for patients with exocrine pancreatic cancer is low, in the range of 5–10%. Current diagnosis for this cancer uses a combination of radiologic procedures and biopsies. An early diagnostic method coupled with a therapeutic counterpart may have a profound effect on survival and quality of life.
Structure-activity relationships have shown that the C-terminal sequence (amino acid residues 8–13 (named NT(8–13))) of the natural neurotensin is sufficient for preserving high affinity receptor binding (Granier et al., 1982; Kitabgi et al., 1985). The affinity of this analog is comparable to that of natural neurotensin in two different binding assays, i.e., the binding assay on rat brain synaptic membranes and on HT 29 cells which express neurotensin receptors. Unfortunately this truncated peptide has poor in vitro stability. One site of enzymatic instability is the Arg8-Arg9 bond which has a serum t2 of 5 minutes. The serum stability was increased with the Lys8(Ψ-CH2NH)Arg9 pseudo-peptide DTPA-Lys8(Ψ-CH2NH)Arg9-Pro-Tyr-Ile-Leu-OH (SEQ ID NO:2) (Tourwe et al., 1998).
Two radioiodinated derivatives of neurotensin are described in the literature. Because there are two tyrosine residues, iodination yields a complex mixture of products. These derivatives are difficult to purify and each one possesses different biological properties. To overcome this problem, Mazella et al. (1983) synthesized monoiodo-125-[Trp11]-neurotensin. This derivative has a Kd of 0.1 nM binding to rat brain synaptic membranes. The same group of researchers later succeeded in preparing the monoiodo-125 derivative of natural neurotensin derivative. This radioiodo analog has a Kd of 0.26 nM for binding on human brain neurotensin membranes.
These iodinated derivatives of natural neurotensin peptides are unsuitable for imaging and therapy of tumors expressing neurotensin receptors because of difficulty in the method of preparation as well as the instability of these derivatives. The instability results from rapid deiodination and also from the enzymatic degradation of the natural neurotensin peptide bonds.
NT(8–13) contains only one Tyr residue which can be selectively radioiodinated. Structure activity studies indicated that the iodination resulted in the loss of binding affinity by a factor of 20.
Since the early work, other radiolabeled neurotensin analogs have been prepared. Tourwe et al. (1998) have prepared diethylenetriamine pentaacetic acid (DTPA)-NT(8–13) (DTPA-Arg-Arg-Pro-Tyr-Ile-Leu-OH (SEQ ID NO:3)) and found that the derivative had an affinity of 6.5 nM to HT 29 colon adenocarcinoma cells. The low tumor uptake in nude mice HT29 tumor was ascribed to the rapid metabolism of the DTPA-NT(8–13). The in vivo half-life of neurotensin is less than 1.5 minutes and the major cleavage site has been shown to be the Arg8-Arg9 bond (Lee et al., 1984; Aronin et al., 1982). Hence, neurotensin analogs in which the peptide bonds were sequentially replaced by Ψ(CH2NH) were prepared and a large drop in affinity was observed. The compounds DTPA-Lys8-Ψ(CH2NH)-Arg8-NT(8–13) and DTPA-Lys8-Ψ(CH2NH)-Lys8-NT(8–13) had a Kd of 13 and 7.4 nM, respectively.
An analysis of the above compounds indicated that the stability of the compounds in the serum is not sufficient for these compounds to be used as radiolabeled imaging and therapeutic agents.
The publications and other materials used herein to illuminate the background of the invention or provide additional details respecting the practice, are incorporated by reference, and for convenience are respectively grouped in the appended List of References.